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Coherent p+ Photoproduction on 3He (Ilieva, McCallum, Berman) In the past, the study of the reaction g3He ® 3Hp+ was aimed at comparing the coherent photoproduction of positive pions on a nucleus with the same process on a free nucleon. However, even improved DWIA calculations deviate by a factor of 100 from experimental cross sections at high momentum transfers [Kam91]. Other production mechanisms, such as two-step processes, have been suggested, but not tested against experimental data. Previous measurements of this process extend only up to beam energies of 450 MeV (maximum momentum transfer of 0.5 (GeV/c)2) [Bac75, Bel84]. Since we have already studied coherent pion photoproduction on the free nucleon and deuterium at energies up to few GeV, it is a natural extension to measure this process and to study high-momentum-transfer phenomena on the three-nucleon bound system. Since there are two charged particles in the final state, we make use of the good CLAS acceptance for charged particles and base the analysis on detecting 3H and p + in coincidence. Various kinematics cuts allow us to extract the particle yields with no background subtraction within an uncertainty of less than 3%. Preliminary CM angular distributions, obtained after analyzing 25% of the whole data set, are shown in Fig. 11. The highest momentum transfer we can access is about 2 (GeV/c)2. This is about the same as in the case of coherent pion photoproduction on the deuteron (Section 5 above), which will allow us to study both the medium modification of the relevant reaction mechanisms and the importance of three-body final-state interactions. As of now, the analysis method for extracting the yields has been studied and finalized. The detector acceptance and inefficiencies as well as the photon flux still need to be calculated. This part of the analysis is in progress. The analysis of this reaction is the subject of the Master’s thesis of Don McCallum of Trent University.
Figure 11. g3He ® 3Hp+ CM yields. Black: 3He angular distributions; red: reversed p+ angular distributions. The photon-energy bins (labeled at the top of each panel) are 100 MeV wide. |
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